1. Field of the Invention
The invention relates to a method for an intersystem handover. In particular, the invention is suitable for use in a mobile radio system or wireless subscriber access system.
2. Description of the Related Art
In radio communication systems (also called radio access technology—RAT—in the text which follows), for example the second-generation European mobile radio system GSM (global system for mobile communications), information such as, for example, voice, image information or other data are transmitted via a radio interface with the aid of electromagnetic waves. The radio interface relates to a connection between a base transceiver station and a multiplicity of subscriber stations, where the subscriber stations can be, for example, mobile stations or stationary radio stations. Electromagnetic waves are radiated with carrier frequencies which are located within a frequency band provided for the respective system. For future radio access technologies, for example the UMTS (universal mobile telecommunication system) or other third-generation systems, frequencies are provided in a frequency band of approx. 2000 MHz. For the third mobile radio generation UMTS, two modes are provided, one mode being an FDD (frequency division duplex) mode and the other mode being a TDD (time division duplex) mode. These modes are used in different frequency bands and both modes support a so-called CDMA (code division multiple access) subscriber separation method.
For descriptions of the second-generation mobile radio system GSM, the book by J. Biala “Mobilfunk und Intelligente Netze” (which may be translated as “Mobile radio and intelligent networks”), Vieweg Verlag, 1995 provides the basic general state of the art.
Due to the fact that the second- and third-generation radio access technologies exist in parallel and it is desired to harmonize between them, subscriber stations which have set up a connection in one radio access technology are to be provided with the possibility of handing the connection over to another radio access technology which may support a different transmission mode. This handing over between technologies is also called intersystem handover.
At the beginning of the spread of the future UMTS mobile radio system, so-called multimode subscriber stations will support both the GSM standard and the UMTS standard at the least. This is mainly of significance to operators which are implementing both, for example, a full coverage of an entire country by the GSM mobile radio system and an initially locally limited coverage by the UMTS mobile radio system.
According to the GSM standard, the subscriber station must observe, for example, the received signal strength indicators (RSSI) of up to 32 neighboring cells and the six neighboring cells offering the best transmission conditions must be periodically signaled to the base transceiver station providing coverage.
In parallel with this observation of neighboring cells of the system to which the subscriber station has set up a connection or is in a so-called idle mode, the subscriber station will also have to additionally observe radio cells of one or more radio access technologies available in parallel in future. On the one hand, this disadvantageously increases the energy consumption of the subscriber station and, on the other hand, a certain time for observing all available systems is required in each case. For this reason, it is basically logical if the subscriber station initially only observes radio cells of one system and an observation of another system is only stimulated if the transmission quality to this system drops below a certain threshold value.
For the GSM standard, it is presently proposed to expand the so-called control parameters in such a manner that two threshold values QSEARCH and QACCEPT are additionally signaled in the control parameters in the downlink. The complete list of control parameters is given in a table in FIG. 2. The proposed threshold values are emphasized in italics.
The first threshold value QSEARCH describes a lower value for the quality of a received GSM signal above which no measurements of further systems (alternate RATs) are performed. The second threshold value QACCEPT describes a lower value for the quality of the signal of the alternate RAT below which a handover to the other system is unacceptable.
FIG. 4 is used for describing how these additional parameters affect the behavior of a subscriber station. Referring to FIG. 1 described hereinafter, values determined in the subscriber station for a transmission quality are plotted against time t. The time-variant first signal sig1 is, for example, the general signaling channel BCCH (broadcast control channel) from a base transceiver station of a GSM mobile radio system, which is transmitted by the base transceiver station with an increased and constant transmit power. The time-variant second signal sig2 is, for example, a general signaling channel BCH (beacon channel) transmitted by a base station of a UMTS mobile radio system. The time variance is mainly due to a movement of the subscriber station, but a change in the traffic loading within the radio cell within which the subscriber station is located can also lead to fluctuations in the transmission quality.
According to the proposal described, the subscriber station begins to observe the second signal sig2 after the first signal drops below the first threshold value QSEARCH and ends this observation after the first threshold value QSEARCH has subsequently been exceeded etc. In this process, the consequence of dropping below the first threshold value QSEARCH and the second signal sig2 simultaneously exceeding the second threshold value QACCEPT would be, for example, that a handover to the second system is stimulated.
The consequence of this behavior is that a frequent short-time starting and stopping of the observation of the second system would occur, particularly when values of the transmission quality are close to the first threshold value. This wastes precious energy, on the one hand, and, on the other hand, the period of observation is too short for being able to draw reliable conclusions for any handover from the measurements of the transmission quality to the second system. In this context, it must be generally taken into consideration that a single measurement of a radio cell of the second system is not sufficient. Instead, a number of measurements must be performed in order to obtain a reliable result.